[in Russian with English abstract: ] In this study, we aim to evaluate the diversity of lichens and allied non-lichenised fungi, co-existing in dark-coniferous and broad-leaved forests of the southern taiga in the East European Plain. The Kologriv Forest State Nature Reserve (589 km2) was established in 2006 to preserve the last fragments of the unique primeval forests of the southern taiga. The territory of the Kologriv cluster of the Kologriv Forest State Nature Reserve (481 km2) is located in the north of the Kostroma Region (Russia). The main phorophytes in this cluster are Picea abies, Tilia cordata, Betula pubescens, and Abies sibirica. In our time-limited study (four days in May 2021), we selected a 10 000-m2 (hereinafter – 1-ha) plot (a presumed lichen diversity hotspot) in the primeval forest stand in the core of the Kologriv cluster of the Kologriv Forest State Nature Reserve (58.80164° N, 43.98533° E, at altitude ca. 200 m a.s.l.). For a detailed study of the species composition of lichens on this plot, all types of substrates were examined, including trunks and branch surface of various tree species under various conditions, dead and rotting wood of snags, stumps, and fallen logs. In addition, 22 points in various parts of the protected core were examined by the route method in May 2019 and May 2021. This detailed survey in the primeval forest in the core of the Kologriv Forest State Nature Reserve has revealed an unexpectedly high lichen diversity: 230 species recorded in a single 1-ha plot is equal to or much more than the number of species recorded in several Protected Areas of far larger sizes in European Russia. The number of species found on this plot is 73% of the one recorded in the whole protected core (316 species) and 58% of the lichen diversity known in the entire area of the Kologriv Forest State Nature Reserve (398 species). The genus Asterophoma is new for European Russia. The genera Bachmanniomyces and Bryostigma are new for the lichen flora of the middle belt of European Russia. There are 91 species listed for the first time for the lichen flora of the Kologriv Forest State Nature Reserve. Of them, 83 species are new to the Kostroma Region. Two species (Micarea synotheoides and Stigmidium exasperatum) are new to Russia. Rinodina macrospora was found for the first time in Europe. Three species, Asterophoma mazaediicola, Biatora veteranorum, and Biatoropsis minuta, are new to European Russia. 13 species were found for the first time in the middle belt of European Russia: Bachmanniomyces punctum, Bryostigma lapidicola, Heterocephalacria bachmannii, Lepra borealis, Micarea byssacea, M. melaeniza, M. nowakii, M. pseudomicrococca, M. pycnidiophora, Ochrolechia mahluensis, Scoliciosporum perpusillum, Skyttea gregaria, Stigmidium congestum, Trapeliopsis gelatinosa, Xylographa soralifera. Four threatened species (Leptogium burnetiae, Lobaria pulmonaria, Menegazzia terebrata and Nephromopsis laureri) are listed in the Red Data Book of the Russian Federation. We have made a comparison of the lichen flora diversity found in the core of the Kologriv Forest State Nature Reserve with previously obtained data in Western and Central Europe. The core of the Kologriv Forest State Nature Reserve meets the criteria of a hotspot of lichen biodiversity on the European level. We would like to emphasise the importance of finding such hotspots of lichen diversity by an inventory of the lichen flora in other Protected Areas. Keywords: biodiversity, inventory, Kostroma Region, lichen flora, Micarea synotheoides, primeval forest, Rinodina macrospora, Stigmidium exasperatum, Usnea longissima, virgin forests.